In a fuel injection engine, a basic fuel injection amount is determined according to an intake air amount each time a cylinder performs an intake cycle, and a fuel injection amount is determined by multiplying this basic fuel injection amount by various correction coefficients. Fuel corresponding to this amount is intermittently injected from a fuel injection valve into the intake air in synchronism with the engine rotation.
The aforesaid coefficients comprise a post startup increase coefficient KAS which is determined by the engine cooling water temperature and the time elapsed since completion of startup. When the engine is cold, some of the injected fuel deposits on the wall surfaces of intake passages and intake valves (referred to hereinafter as deposited fuel). Part of the deposited fuel evaporates, but part flows down the walls and through the intake valves into the combustion chamber so as to set up what is referred to as wall flow. For this reason, when the engine is cold, the fuel entering the combustion chamber tends to be delayed, and this delay is compensated by increasing the amount of injected fuel using the post startup increase coefficient KAS.
Immediately after startup is completed, an initial value of KAS is used, and a gradually decreasing value of KAS is applied as the elapsed time increases. This characteristic is experimentally determined so that emission of noxious components of exhaust gas, i.e. HC and CO, do not increase, and good drivability is maintained.
However, the initial value of the post startup increase coefficient KAS is determined according to engine cooling water temperature during startup, and no account is taken of how much deposited fuel remains in the intake passage and intake valve when startup is completed. Consequently, the initial value of KAS may be too large or too small depending on the time taken for the engine to start, increasing emission of HC or CO and adversely affecting drivability.
For example, when engine startup is completed in a short time, the fuel amount injected during startup is less, and the amount of fuel deposit when startup is completed is less than the equilibrium fuel deposit amount. The equilibrium fuel deposit amount is the amount of fuel deposited when the vehicle is running under steady state conditions, and this amount depends on the temperature of the part where fuel is deposited. When engine startup 1s completed in a short time, therefore, a considerable part of the fuel injected immediately after startup is completed, deposits on the wall surfaces of the intake passage or intake valve. It is thus necessary to set the initial value of KAS large.
On the other hand, when a long time is required for startup, more deposited fuel remains when startup is complete than the set value, and if the initial value of KAS is set large as in the aforesaid case, the fuel injection amount is excessive so that emission of HC and CO increases.